vga signal amplifier

BobW · Dec 10, 2015

Okay thanks. That seems to settle the issue then. It looks like you're back to having to build an amplifier.

lynx · Dec 10, 2015

BobW said:
Okay thanks. That seems to settle the issue then. It looks like you're back to having to build an amplifier.

also...no matter how many resistors i add between the sync pins and the gnd the result is the same...
i tried up to 150 ohm.

if i need to build an amplifier then it makes my initial idea correct...but some of you said that the monitor will
compensate the amplification and the outcome will be as it is now...

no?

ccurtis · Dec 10, 2015

Ron asked a while back if you tried another cable. Simple things first. I've come across some pretty crappy VGA cables in my experience, giving bad video.

ronsimpson · Dec 10, 2015

Lynx,
If I have the right manual;
Have you played with "MagicBright"?
OR
"Colour Tone"?
There is a way to make the monitor look the same as your printer. If that is programmed it might make the monitor dark.
God knows what MagicBright is but try playing with that adjustment. "only works in PC mode"

lynx · Dec 11, 2015

ccurtis said:
Ron asked a while back if you tried another cable. Simple things first. I've come across some pretty crappy VGA cables in my experience, giving bad video.

no i didn't because the cable is new, but i will

ronsimpson said:
Lynx,
If I have the right manual;
Have you played with "MagicBright"?

yes i did as i said in the following post..

lynx said:
- useless (atleast to me) "magicbrightness" future.

lynx · Dec 11, 2015

tried a cable from an EIZO brand display which has feritte beads on it, etc and the result was the same.

AnalogKid · Dec 11, 2015

ronsimpson said:
The problem with AGC is that, in TV video there is no guarantee the video will have "white" in it.

In the mid to late 70's, back when video was real video, the FCC OK'd broadcasting VITS and VIRS, the vertical interval Test and Reference signals. VIRS had everything a smart receiver needed to correct for differential gain, differential phase, absolute chroma phase, black level, and peak white level, all independent of the program content. All moot in the digital broadcast world.

ak

BobW · Dec 11, 2015

Television uses negative video, where white is zero volts. Hence, there's always a white reference. White and black reference levels were never a problem for television. That was designed into the system from day 1. However, the VITS and VIRS signals were a big benefit for color TV because prior to that, there had been no color reference signals.

ronsimpson · Dec 11, 2015

BobW said:
Television uses negative video, where white is zero volts. Hence, there's always a white reference.

I have heard this, many time above in this thread. Will some one please show me a picture of TV video where white is at zero volts.
I agree white is negative compared to black.

audioguru · Dec 11, 2015

TV video was transmitted with AM modulation. Sync was (blacker than black) full power, black and blanking were a little less at 76% power and white was 20% power.

White cannot be zero power because then the audio would stop whenever the video had white.
Black was always on every front and back porch so black had a good reference voltage and the contrast control adjusted the brightness of whites. The brightness control adjusted how much light there was in blacks (grey?).

BobW · Dec 11, 2015

I'll backpedal. Turns out that for NTSC negative video, "peak white" is at the 12.5% level, not 0% as I stated earlier. See link:
https://www.broadcaststore.com/pdf/model/793698/TT148 - 4053.pdf

However, whether white is 0% or 12.5%, it really changes nothing as far as being able to restore proper contrast and black level from an attenuated signal. We know that black is 0.75 volts on a standard 1 Vp-p signal and white is at 0.125 volts. If the signal has been attenuated and we read black to be 0.6 volts, then we know that the signal has been attenuated to 80% (0.6/0.75) of its nominal value, and we know that white is now 0.8*0.125 = 0.1 volt. Hence, it's always possible to restore both the original black level and original contrast. And, this has been standard procedure for broadcast television since its inception.

Whether a VGA monitor makes use of this information is another matter entirely.

ronsimpson · Dec 11, 2015

BobW said:
Whether a VGA monitor makes use of this information is another matter entirely.

I can not think of a VGA monitor that uses AGC.

BobW said:
However, whether white is 0% or 12.5%, it really changes nothing as far as being able to restore proper contrast and black level from an attenuated signal.

I think AGC is not done to video.
The AGC is done at the RF amp, and at the IF stages but not done after the detector. The variable gain amp is before the detector.
Monitors with out (a radio receiver) do not have AGC.

BobW · Dec 11, 2015

Fair enough. I only brought this up in the first place because I was concerned that if monitors did use some form of AGC, then simply amplifying the overall signal wouldn't achieve the desired increase in contrast. Obviously, this is not the case, and Lynx's tests with the resistors have confirmed it.

lynx · Dec 12, 2015

is it sure if i build the amplifier it will work?

thanks.

ronsimpson · Dec 12, 2015

lynx said:
is it sure

No. Life is not sure.

I thought about a simple circuit that will pull low during sync and high for not sync. With a pot we can set the high level.
BUT
With out a o-scope it probably is not worth trying.
The front end of the monitor has a ADC. It is set to work over a certain range. I don't know the range but I know if the signal is too big the signal will be "clipped".
You could build one amp on only one channel. Bread board. If the amp is not fast enough that is ok for now, just to see if you get a brighter picture.

AnalogKid · Dec 12, 2015

ronsimpson said:
I have heard this, many time above in this thread. Will some one please show me a picture of TV video where white is at zero volts. I agree white is negative compared to black.

I started in TV repair, where white was down and black was up because that's the way the transmitter is modulated. Then I worked in broadcast television where *everything* is white-up, black-down. The only place the video is inverted is inside the transmitter. The transmitter puts peak power into the sync pulse so that no matter now noisy the signal is at the receiver, it is stable on the screen.

Comment to audioguru - I don't think so. The baseband video signal and the audio-modulated FM subcarrier are independent signals that are combined in the exciter. No matter what the video does (assuming the transmitter isn't WAY out of alignment), the transmitter continues to broadcast the FM audio subcarrier modulated on the AM channel carrier.

ak

BobW · Dec 14, 2015

I don't see any need for an op amp. A simple transistor amp like a common emitter stage possibly followed by an emitter follower should give enough gain and have plenty of bandwidth. Certainly good enough for testing. And as Ron says, you can test it on just one signal.

AnalogKid · Dec 14, 2015

A simple common-emitter stage, with or without an emitter follower for output impedance control, will have significant (visibly noticeable) differential gain and phase errors.

ak

audioguru · Dec 14, 2015

Many years ago when I went into video conferencing I made a video distribution amplifier with ICs designed to do it and it worked perfectly. A transistor would have made a distorted mess of it.

AnalogKid · Dec 14, 2015

My first experience with video distribution amplifiers (DAs) was installing and calibrating upper middle class devices in the TV station I was installing. Broadcasting was starting to accept opamps for audio, but video ICs were years away. The DAs were an all discrete version of a classic opamp layout, diff amp, voltage amp, constant current source pull down in a class A output stage, and a +24 V / -12 V power rails for a 2 V output. They made very good pictures.

I did a character generator output stage with an LM318, but that was mono.

ak

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